The present invention relates to data communications equipment and, more particularly, to a communications system involving simultaneous voice and data modems.
The co-pending, commonly assigned, U.S. Patent application of Bremer et al. entitled "Simultaneous Analog and Digital Communication," Ser. No. 8/076505, filed on Jun. 14, 1993, U.S. Pat. No. 5,448,555, describes a simultaneous voice and data communications system in which a voice signal is added to a data signal for transmission over a communications channel to a receiving modem.
In this simultaneous analog and digital communication system, the data signal to be transmitted is represented by a sequence of data symbols, where each data symbol is associated with a particular N-dimensional signal point value taken from a signal space. Similarly, the analog signal, which is represented by a voice signal, is processed so that it is mapped into the N-dimensional signal space to provide a voice signal point. This voice signal point defines the magnitude and angle of a voice signal vector about the origin of the signal space. The data symbol and the voice signal vector are then added together to select a resultant N-dimensional signal point, which is then transmitted to a far-end modem.
Upon reception of the transmitted N-dimensional signal point, the receiver of the far-end modem detects the embedded data symbol and subtracts the data symbol from the received N-dimensional signal point to yield the voice signal vector. This voice signal vector is then used to recreate the voice signal.
As a result, this technique advantageously provides a voice-band signal that has both an audio channel and a data channel. This allows two users with simultaneous voice and data (SVD) capable modems to communicate data between them and talk at the same time--yet only requires one "tip/ring" type telephone line at each user's location. As in any communications system that includes a voice signal, the characteristics of the SVD signal are normally chosen to optimize the voice quality of the SVD voice channel. That is, higher quality voice transmission is sought. For example, the co-pending, commonly assigned, U.S. Patent application of Bremer et al., entitled "Companding of Voice Signal for Simultaneous Voice and Data Transmission," Ser. No. 08/076506, filed on Jun. 14, 1993, describes a simultaneous voice and data communications system in which a voice signal is companded before application to the PSTN to improve the quality of the voice transmission.